This post is perfect! I wish I could like it 3 times over. There's some other great advice on this thread.
I've just received my results, and I got 11 9s (including L2 Further Maths). The best tip I could give is to
learn everything right the first time. There's a reason revision is called "revision" -- you're going over information you've already learnt! Pay attention in lessons. When I was bad at a subject, I would make more of an effort to put my hand up when I
did know the answer to boost my confidence (and reduce the chance of the teacher picking on me...), which really worked! Saying things allowed or when asked a question helps you remember better. It can be embarrassing to get it wrong, but at the end of the day, it's about you and your grades. If you participate enough, the teacher might even think you're joking

If you do use flash cards, wording them as questions is very effective. Make sure to prioritise the beginning information and key words for the flash cards, considering these are the fundamentals. Next, put what you struggle with most onto flash cards. Brainscape is good because you can rate your knowledge from 1 to 5. Reading the specification is very important for the sciences, and you can even take phrases directly from it, change them into a question and put the answer. (A few came up in my chemistry paper!)
Another great tip:
Take your mocks seriously! These are the best practice for how it will feel in a real exam hall. If you don't bother revising for French in the mock, for example, it'll only make you more worried for the real exam. This is your best opportunity to test revision techniques and see how they work.
I have loads to say about languages (239/240 in French and 231/240 in German, which I started in Year 10), so reply if you want advice on that

I can talk about NEA subjects as well! (I did food tech

)
Getting enough sleep is very important. The only time I didn't sleep on time was when I was staying up until almost 3am revising history – I
don't recommend this, given that I couldn't focus properly in my maths paper 2 (but it was my best history exam). Being well-rested helps you remember more information and allows you to work out and process the question better, which is important for all subjects!
When exam season approaches, DO NOT revise for 12 hours a day for 3 months – it's not sustainable, and it's not needed. I revised for maybe 4 hours a day with good quality revision instead of re-reading. If you make flash cards or do past papers, more time will be needed than that, but resting enough is essential. I was relatively kind about taking breaks, and instead of a strict revision schedule, I had tasks I wanted to achieve that day to avoid a feeling of, "Oh, I was supposed to start at 5pm and it's 17:03..."
Lastly, I'll say that getting all 9s isn't easy, and it does require a trade-off between spending more time on subjects you're struggling with at the expense of subjects you're great at. I am, and always will be, eternally grateful for my English teacher going over essays with me after school to help me perfect my exam technique 2 days before my first English Literature exam.
PS: Revision resources I used were Seneca, Cognito for the sciences, Brainscape, the specifications, YouTube videos for subjects like history, DystopiaJunkie on YouTube for WJEC/Eduqas English, The GCSE Maths Tutor on YouTube, Memrise for the languages,
many past papers, Duolingo Stories before they did that random update, and PMT (Physics and Maths Tutor). I never paid a penny, but I heard CGP books are great!